By default there is a Windows share called etc$. From a windows machine browse to \\filerhostname\etc$, for the username try filerhostname\administrator, and for the password, use the root password of your netapp.
Or use the command line:
priv set advanced
<Cut and paste output into a text editor, make your modification, and copy the ENTIRE contents to the clipboard>
<paste the entire contents of your exports file>
rdfile /etc/exports (to confirm the contents are correct)
The key here is that wrfile can only be used to write the entire file. You can't modify or append using wrfile.
Another option to modify this file would be to mount up the root vol on a UNIX box using NFS.